thermal: imx_sc_thermal: Fix incorrect data type

The temperature value passed from SCU could be negative value,
the data type should be signed instead of unsigned.

Fixes: e20db70d ("thermal: imx_sc: add i.MX system controller thermal support")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1584606380-9972-1-git-send-email-Anson.Huang@nxp.com